Close() calls shared.close() first, which marks the watcher as closed
by closing the done channel. It then iterates calling Remove() for
each watched path. But remove() checks isClosed() at entry and
short-circuits without closing any file descriptors.

This leaks every file and directory watch descriptor opened by
addWatch(). On macOS with kqueue, watching a directory also watches
all files within it, so a watcher monitoring N directories with M
total files leaks N+M descriptors per Close() call. Long-running
processes that recreate watchers (e.g. dev servers with hot reload)
accumulate leaked descriptors until hitting EMFILE.

Fix: close all watch descriptors directly by iterating the wd map
instead of delegating to Remove().

Signed-off-by: Christian Stewart <christian@aperture.us>